The object of this study is to develop an in vitro potency assay to replace the guinea pig death test portion of the official release test for the tetanus component of adsorbed toxoid vaccine. ER2 (Guinea Pig ELISA standard 10 units) was used in the Tetanus ELISA assay to determine the titer of routine lot release serum. Sera from DTP, adsorbed, lots were saved, because they were borderline samples in the official potency assay. Addition samples were collected to calculate both the individual and pool titers of each new DTP, adsorbed immunized guinea pig. The pools were tested in the official potency assay. The individual sera and pools were tested in the ELISA assay. The correlation between the official potency assay and the ELISA was good. A US Standard Antitetanus (E134 and E135) was compared to Guinea Pig serum (ER2 and ER3 the alternate ELISA standard 8 units) in the Tetanus ELISA. There is approximately 200 fold difference between the calculated units in ELISA for Guinea Pig serum and US Standard (horse), when using the unit value from the animal potency assay. These results are being evaluated to determine the feasibility of using the US Standard with a multiplier. If the US Standard cannot be used, in house standards made from Guinea Pig serum and validated against the US Standard will be recommended. The Standard Operating Procedure and samples of the tetanus toxin and Guinea Pig control sera have been sent to one of the licensed manufacturers. Data will be exchanged and functional characteristics will be examined.